I had a feeling it was the gasket...a lot of the time a worn, broken or wrong gasket will do that. A lot of people think it's because of a faulty FITV or any of the other associated sensors (O2 sensor won't give u that prob, just shit economy and performance). ALWAYS check the TB gasket as well before buying new sensors...when replacing, make sure to WD40 it before fitting it so it leaves no leaks.
When dealing with hunting issues I always start by checking for air/vacuum leaks.
Start by running the car and using some brake or throttle body cleaner (anything that won't damage components but is highly combustible) and spray liberally and slowly making your way over the entire inlet tract with spray (paying special attention to [be more liberal with the spray] intake pipe and inlet manifold fittings).
If the idle changes (increases breifly) you should be paying attention to the area where you were spraying once the idle increased. Spray over the area again to pin-point the component/part requiring attention (leaking air) and then act upon the leak - fix/repair/replace it.
If no vacuum leaks are detected - then move onto further diagnosis.
The way this works is that the combustible spray gets sucked in (via the vacuum leaking component) and is 'un-metered' so the idle increases briefly. This gives you an audible/visual sign that there is a vacuum leak present - causing your rough running/hunting issue.
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